Question 21 markMCQSection A

$Which of the following is not a component of immune system ?$

Q: Which of the following is not a component of immune system ?

**Correct option: C — Red Blood Cells** Red Blood Cells (erythrocytes) transport oxygen and carbon dioxide; they play **no role in immune defence**. - Lymphocytes, plasma proteins (antibodies/complement) and bone marrow are all part of the immune system. - RBCs lack a nucleus and immune function, so they are the odd one out.

Human Health and Disease

Components of the Immune System

Options

(A)

Lymphocytes

(B)

Plasma Proteins

(C)

Red Blood Cells

(D)

Bone Marrow

Official answer

Correct option: C — Red Blood Cells Red Blood Cells (erythrocytes) transport oxygen and carbon dioxide; they play no role in immune defence . Lymphocytes, plasma proteins (antibodies/complement) and bone marrow are all part of the immune system. RBCs lack a nucleus and immune function, so they are the odd one out.

immune system

red blood cells

lymphocytes

plasma proteins

bone marrow

lymphoid organ

antibodies

Marking scheme

1. $1 mark: correct option C (Red Blood Cells).$
2. $Full mark awarded only for the correct choice.$

Hint

$Which blood component only carries oxygen and has no defence role?$

Quick oral answer

$Red blood cells are the answer — they only transport respiratory gases; lymphocytes, plasma proteins like antibodies, and bone marrow are all genuine immune-system components.$

Common mistakes

1. $Assuming all blood cells and proteins are immune components — RBCs only transport gases.$
2. $Not recognising bone marrow as a primary lymphoid organ where lymphocytes are formed.$
3. $Overlooking that plasma proteins include antibodies and complement, which are immune molecules.$

Analysis and explanation

Concept The immune system comprises lymphoid organs, immune cells, tissues and soluble molecules that defend the body against pathogens. Why C is correct Red Blood Cells are respiratory transport cells (carry O2/CO2 via haemoglobin). They are anucleate in mammals and have no defensive/immune role, so they are NOT a component of the immune system. Why the distractors are wrong Lymphocytes — B-cells and T-cells are the central cells of specific (acquired) immunity. Plasma Proteins — include antibodies (immunoglobulins) and complement proteins, key humoral defence molecules. Bone Marrow — a primary lymphoid organ where all blood cells, including lymphocytes, originate and B-cells mature. Exam trap Because RBCs, WBCs and plasma proteins all circulate in blood, students wrongly assume RBCs are immune cells. Only WBCs (leucocytes) and their products participate in immunity.

Interesting facts

$Bone marrow and the thymus are the two primary lymphoid organs; spleen, lymph nodes and MALT are secondary lymphoid organs.$

$A single millilitre of blood contains billions of RBCs but they are removed from immune roles entirely — even lacking a nucleus in mammals.$

Question details

Q2 of 37 questions

Board: CBSE
Class: Class 12
Subject: Biology
Year: 2026
Section: Section A
Marks: 1
Type: MCQ
Chapter: Human Health and Disease
Topic: $Components of the Immune System$
Bloom's level: remember
Syllabus topic: $Unit VIII: Biology in Human Welfare — Human Health and Disease: Immunity and Lymphoid Organs$
Author: CBSE
Reviewed by: Padhantu Academic Team

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Frequently asked questions

$Why are red blood cells not part of the immune system?$

$RBCs are specialised for transporting oxygen and carbon dioxide using haemoglobin. In mammals they lack a nucleus and have no capacity for phagocytosis, antibody production or antigen recognition, so they are excluded from immune defence.$

$Is bone marrow really part of the immune system?$

$Yes. Bone marrow is a primary lymphoid organ. All blood cells, including immune lymphocytes, are produced here, and B-lymphocytes complete their maturation in the bone marrow.$